In a typical tomosynthesis system, an x-ray source emits x-ray beams in the direction of an object to be imaged. These x-ray beams are subsequently detected by a detector, and are acquired by the tomosynthesis system.
Further, in a typical tomosynthesis system, the imaged object remains in a fixed position, the x-ray source assumes different discrete positions along a trajectory in space. For each source position, a projection radiograph image is acquired.
Once the images, included in the detected x-ray beams, of the object are acquired by the tomosynthesis system, reconstruction of three-dimensional structures of the imaged object from structures included in the acquired images is accomplished by the tomosynthesis system. Accurate and reliable reconstruction of 3-dimensional information about structures of the imaged object in tomosynthesis requires a precise knowledge of the tomosynthesis system geometry. The relative positions of the focal spots of the x-ray source for the different x-ray source locations, and the detector at the time the images of the object are being acquired is referred to as the tomosynthesis system geometry. The focal spot is the location of the point-like approximation of the x-ray source. The focal spot is typically at a fixed location with respect to the elements of the x-ray source. For the purposes of reconstruction, the focal spot represents the location of the x-ray source. This precise knowledge of the tomosynthesis system geometry is required in order to obtain a high image quality and a high level of detail-resolution of reconstructed structures in the imaged, 3-dimensional object.
One conventional method of determining the tomosynthesis system geometry is by physically measuring the spatial location and orientation of the different components (the detector and the focal spot positions of the x-ray source at the different source locations). This method requires a number of precise measurements different source locations). This method requires a number of precise measurements and is typically complex, depending on the mechanical structure which connects the detector and the x-ray source.
Moreover, other conventional methods can be used that are similar to calibrating an internal camera geometry using images of scenes with unknown geometry. Although conceptually similar to the problem of determining the tomosynthesis system geometry from images of some object, these other conventional methods cannot be carried over to determining tomosynthesis system geometry because a tomosynthesis system includes a different system geometry for each acquired image, typically because of a change in the x-ray source (and thus focal spot) location for each acquired image.
In computed tomography (CT), there exist approaches that employ markers (or phantoms) with a well-defined relative position of the markers. However, these approaches are based upon the use of a well-defined phantom (which is relatively expensive), and usually cannot be used while imaging a patient. Consequently, the imaging procedure has to be repeated if the CT system geometry was not well calibrated during the patient image acquisition.